In a distance measuring apparatus which measures a propagation time for a radio wave from a transmitter to a receiver, and calculates the distance, it is necessary to measure the reception time of the radio wave in the receiver.
As a method of measuring the reception time of the radio wave, heretofore there is a method of measuring the reception time using a spread spectrum communication (for example refer to Marubayashi et. al. “Spread Spectrum Communication and its Application”, The Institute of Electronics, Information and Communication Engineers”). Hereunder is a brief description of the case of using a direct diffusion process using a PN code.
The transmitter diffusion processes the data signal using the PN code, and generates a diffusion signal of a baseband frequency band, and modulates this signal and transmits it as a radio signal of an RF (radio frequency) band. The receiver demodulates the received radio signal to a demodulation signal of the baseband frequency band, and inverse diffusion processes the demodulation signal with a matched filter using the PN code. The inverse diffusion output which is output from the matched filter shows a maximum value when the demodulation signal is in the same phase as the PN code of the matched filter, and if the phase is displaced by one chip or more of the PN code this becomes approximately zero giving a triangular waveform shape. For example the generation time of the maximum value of the triangular wave is measured and made the reception time.
Furthermore, there is a device which obtains the reception time difference of the radio wave, and measures the distance (for example Japanese Unexamined Patent Publication No. 2001-36943).
This receives the carrier wave transmitted from the vehicle side with an on route radio, and modulates the carrier wave received by the radio, based on the spread spectrum code, and replies to the vehicle side. On the vehicle side, the modulated wave from the on route radio is respectively received by the plurality of receivers, and the carrier wave component is removed from the received modulated wave using a band pass filter, and the frequency components of the spread-spectrum codes are respectively extracted, and the extracted spread-spectrum codes are demodulated to obtain the phase difference (equivalent to the reception time difference), and from this phase difference, the difference between the receivers of the vehicle and the on route radio is calculated to thereby specify the vehicle position.